Pongo pygmaeus wurmbiiis one of the three Bornean Orangutan subspecies currently recognized (Goossens et al. 2009). This taxon is found only in Indonesia, in the provinces of West, Central and East Kalimantan.

The estimated decline of the Southwest Bornean Orangutan subspecies has been over 50% during the past 40 years – less than two generations (Meijaard et al. 2011), and many small populations have already disappeared (Abram et al. 2015). The predicted rate of development in Indonesia will result in the destruction of more than half of the current orangutan range in the next 50 years or so, primarily because of forest loss due to conversion to agriculture and to fires (Wich et al. 2012). In most of the Southwest Bornean Orangutan's range, illegal hunting – for bushmeat, the pet trade and because of conflicts – remains a major factor in their decline (Davis et al. 2013, Abram et al. 2015). In sum, the subspecies will undergo > 86% decline in three generations (1950–2025) hence it qualifies for listing as Critically Endangered. See species-level assessment for further details (Pongo pygmaeus).

The total number of Southwest Bornean Orangutans is still poorly known, but most of their range occurs outside protected areas. For a long time, several protected populations – Sebangau National Park and surrounding areas, home to the largest orangutan population in Borneo (Wich et al. 2008), Tanjung Puting National Park and Gunung Palung National Park – were perceived as being stable. However, despite their protected status, these populations are threatened by illegal logging, fire and conversion to agriculture, with the establishment of plantations and agroforests in and at the boundaries of these protected areas (Wich et al. 2012; Gaveau et al. 2013). Fires are a major threat: over 4,000 km2 of Sebangau peatland forest were burnt in 1997–1998, which resulted in an estimated loss of 8,000 orangutans in this population alone (Husson et al. 2015). Hunting, which is illegal, is also perceived as a real risk to sustaining the unprotected populations in the long term: all populations are expected to decline even further in the next 50 years (Abram et al. 2015).

Bornean Orangutans are the largest arboreal mammals in the world, although they walk significant distances on the ground (Ancrenaz et al. 2014). Historically, Bornean Orangutans were most abundant in inundated and semi-inundated lowland Dipterocarp mosaic forests, where movement between different habitat types could buffer them against shortages in food availability in a particular habitat type. Their diet consists primarily of fruits, but also includes leaves, barks, flowers and insects (Russon et al. 2009).

Bornean Orangutans live a semi-solitary life and rarely aggregate in groups. Males are the dispersing sex: upon reaching sexual maturity (at 10–12 years old), they leave the area where they were born to establish large territories covering several hundred hectares. Females’ territories are smaller, with actual size depending on forest type and availability of food resources. Bornean Orangutans are very slow breeders and produce on average one offspring every 6–8 years, which explains their extreme sensitivity to hunting pressure. Females reach maturity at 10–15 years old; they generally give birth to a single infant after a gestation period of approximately 254 days (Kingsley 1981).

Habitat loss. Between 2000 and 2010, the mean annual rate of deforestation for Borneo was 3,234 km2 per year (Gaveau et al. 2014). Assuming a similar deforestation rate in the future, 32,000 km2 of forest could be lost by 2020; 129,000 km2 by 2050 and 226,000 km2 by 2080 (Wich et al. 2015). In the early 2010s, only 22% of the current Bornean Orangutan distribution was located in protected areas (Wich et al. 2012). Approximately a third of the entire Bornean Orangutan range was in commercial forest reserves exploited for timber, and about 45% was in forest areas earmarked for conversion to agriculture or other land uses. A business-as-usual scenario, whereby non-protected forests would be converted along the lines of current development plans, will result in the loss of more than half of the current orangutan range on the island of Borneo in the next 50 years or so.

Illegal hunting. Illegal killing of Bornean Orangutans is a major cause of their decline. Recent interview surveys conducted in Kalimantan revealed that several thousand individuals are killed every year for meat consumption, as a way to mitigate conflict, or for other reasons (Davis et al. 2013). Overall Bornean Orangutan mortality rates in Kalimantan seem to significantly exceed the maximum rates that populations of this slow-breeding species can sustain (Marshall et al. 2009, Meijaard et al. 2011). If hunting does not stop, all populations that are hunted will decline, irrespective of what happens to their habitat. These findings confirm that habitat protection alone will not ensure the survival of orangutans in Indonesian Borneo, and that effective reduction of orangutan killings is urgently needed.

Fires. Fires occur in Borneo on a yearly basis and are responsible for significant forest loss with dramatic results for certain orangutan populations. For example, 90% of Kutai National Park was lost to massive fires in 1983 and 1998 and its Bornean Orangutan population was reduced from an estimated 4,000 individuals in the 1970s to a mere 600 (Rijksen and Meijaard 1999); over 4,000 km2 of peatland forest in southern Kalimantan was burnt to ashes in six months of 1997–1998, resulting in an estimated loss of 8,000 orangutans. In 2015, more than 20,000 km2 of forest were lost to fires, which resulted in hundreds (or more) of additional orangutan deaths.

Habitat fragmentation. With the current scale of habitat exploitation and forest conversion to other types of land uses in Borneo, only a small percentage of current orangutan habitat will remain undisturbed by infrastructure development by 2030 (Gaveau et al. 2013). Several orangutan PHVAs have shown that Bornean Orangutan populations of fewer than 50 individuals are not viable in the long term (Marshallet al. 2009), and that many small populations will go extinct unless they are actively managed (Bruford et al. 2010).

Lack of awareness. A recent study suggested that 27% of the people in Kalimantan did not know that orangutans are protected by law (Meijaard et al. 2011). Campaigns to effectively inform the public and encourage rural people to support the principles of environmental conservation and be actively responsible for the management of their resources are therefore a crucial requirement for successful orangutan conservation.

Climate change. Spatial models point to the possibility that a large amount of current orangutan habitat will become unsuitable because of changes in climate (Struebig et al. 2015). Across all climate and land-cover change projections assessed in a recent analysis, models predicted that 49,000–83,000 km2 of orangutan habitat will remain by 2080, reflecting a loss of 69–81% since 2010. This projection represents a three to five-fold greater decline in habitat than that predicted by deforestation projections alone. A major reduction in the extent of suitable orangutan habitat can be expected. However, core strongholds of suitable orangutan habitat are predicted to remain to the west, east and northeast of the island where populations of P. p. wurmbii and P. p. morio are found.

The Bornean Orangutan is fully protected in Malaysia and Indonesia, and is listed on Appendix I of CITES. However, its forest habitat is not necessarily protected: about 20% of the current orangutan range in Sabah, and 80% in Kalimantan is not protected (Wich et al. 2012). Innovative mechanisms to ensure the long-term survival of Bornean Orangutans outside protected forests are urgently needed.

The future of Bornean Orangutans will very much depend on the long-term security of large, strictly-protected forests where illegal logging and hunting will be efficiently controlled and the orangutan populations large enough to cope with catastrophic events such as fires and disease outbreaks (Meijaard et al. 2011). These forests need to contain the ecological gradients that will provide the key resources to sustain orangutans through climate and other gradual environmental changes (Gregory et al. 2012). In the larger landscape, scientifically-based, regional land-use planning is needed to delineate zones of interaction around protected forests and their surroundings, encompassing hydrological, ecological and socio-economic interactions. Ideally, the core protected areas will remain connected to other areas of forest that could be used sustainably for (commercial) timber extraction. The design of such living landscapes must be approached across the whole landscape rather than at the site level.

In-Place Research, Monitoring and Planning Action Recovery plan:No Systematic monitoring scheme:NoIn-Place Land/Water Protection and Management Conservation sites identified:Yes, over part of range Occur in at least one PA:Yes Percentage of population protected by PAs (0-100):41-50 Invasive species control or prevention:Not ApplicableIn-Place Species Management Harvest management plan:NoIn-Place Education Included in international legislation:Yes Subject to any international management/trade controls:Yes